Unlike the preterite, there are only three verbs that are irregular in the imperfect. These verbs are ir (to go), ser (to be) and ver (to see). Their conjugations are
iba, ibas, iba, íbamos, iban;
era, eras, era, éramos, eran; and
veía, veías, veía, veíamos, veían respectively.
All the other verbs are regular in the imperfect. These are their endings, which replace the endings of the infinitive –ar, er and –ir.
habl–ar: –aba, –abas, –aba, –ábamos, –aban.
com–er: –ía, –ías, –ía, –íamos, –ían.
sent–ir: –ía, –ías, –ía, –íamos, –ían.
Notice that –er and –ir verbs share the exact same endings.
The imperfect is used to narrate habitual or repeated actions that were happening at a certain frame of time in the past without mentioning when they started or when they ended. Therefore, unlike the preterit, the imperfect describes actions, states and conditions whose completion is not seen as an issue. This is the key difference between the uses of the preterit and the imperfect. Whereas the former describes actions which are considered as completely finished, the latter describes actions which lasted for a while but whose ending is not specified. This is a very clear way to distinguish between the uses of these two tenses.
In the same way that the preterit is accompanied by expressions that freeze an action in a specific time in the past (ayer, la semana pasada, hace dos horas, el año pasado, etc.), the imperfect is also accompanied by expressions such as, de niño/a (as a child), de mayor (as an adult), todos los días (every day), mientras (while), siempre (always), which free an action from a specific ending time, and in doing so, such an action remains suspended in the past.
Compare these examples:
Ayer fui a la biblioteca a estudiar. (preterit)
Yesterday I went to the library to study.
De niña, iba a la biblioteca a estudiar. (imperfect)
As a child, I used to go to the library to study.
El sábado pasado comí en casa de mis abuelos. (preterit)
Last Saturday I had lunch at my grandparents’.
Todos los sábados comía en casa de mis abuelos. (imperfect)
Every Saturday I had lunch at my grandparents’.
La semana pasada trabajé sesenta horas. (preterit)
Last week I worked sixty hours.
De joven, yo trabajaba en una librería todos los sábados. (imperfect)
As a young man, I worked at a bookstore.
Anoche mi hermano lavó los platos. (preterit)
Last night my brother washed the dishes.
De joven, mi hermano siempre lavaba los platos. (imperfect)
As a young man, my brother would always wash the dishes.
Este fin de semana, leí una novela de quinientas páginas. (preterit)
This weekend, I read a five hundred-page novel.
De adolescente, siempre empezaba muchas novelas, pero nunca terminaba ninguna. (imperfect)
As a teenager, I began a lot of novels, but I never finished any.
El domingo, jugué al fútbol con mis amigos. (preterit)
On Sunday, I played soccer with my friends.
De niño, siempre jugaba al fútbol con mis amigos. (imperfect)
As a kid, I would always play soccer with my friends.
Ayer por la noche vi una película. (preterit)
Yesterday evening I watched a movie.
Todos los domingos veía una película con mis padres. (imperfect)
On Sundays I used to watch a movie with my parents.
iba, ibas, iba, íbamos, iban;
era, eras, era, éramos, eran; and
veía, veías, veía, veíamos, veían respectively.
All the other verbs are regular in the imperfect. These are their endings, which replace the endings of the infinitive –ar, er and –ir.
habl–ar: –aba, –abas, –aba, –ábamos, –aban.
com–er: –ía, –ías, –ía, –íamos, –ían.
sent–ir: –ía, –ías, –ía, –íamos, –ían.
Notice that –er and –ir verbs share the exact same endings.
The imperfect is used to narrate habitual or repeated actions that were happening at a certain frame of time in the past without mentioning when they started or when they ended. Therefore, unlike the preterit, the imperfect describes actions, states and conditions whose completion is not seen as an issue. This is the key difference between the uses of the preterit and the imperfect. Whereas the former describes actions which are considered as completely finished, the latter describes actions which lasted for a while but whose ending is not specified. This is a very clear way to distinguish between the uses of these two tenses.
In the same way that the preterit is accompanied by expressions that freeze an action in a specific time in the past (ayer, la semana pasada, hace dos horas, el año pasado, etc.), the imperfect is also accompanied by expressions such as, de niño/a (as a child), de mayor (as an adult), todos los días (every day), mientras (while), siempre (always), which free an action from a specific ending time, and in doing so, such an action remains suspended in the past.
Compare these examples:
Ayer fui a la biblioteca a estudiar. (preterit)
Yesterday I went to the library to study.
De niña, iba a la biblioteca a estudiar. (imperfect)
As a child, I used to go to the library to study.
El sábado pasado comí en casa de mis abuelos. (preterit)
Last Saturday I had lunch at my grandparents’.
Todos los sábados comía en casa de mis abuelos. (imperfect)
Every Saturday I had lunch at my grandparents’.
La semana pasada trabajé sesenta horas. (preterit)
Last week I worked sixty hours.
De joven, yo trabajaba en una librería todos los sábados. (imperfect)
As a young man, I worked at a bookstore.
Anoche mi hermano lavó los platos. (preterit)
Last night my brother washed the dishes.
De joven, mi hermano siempre lavaba los platos. (imperfect)
As a young man, my brother would always wash the dishes.
Este fin de semana, leí una novela de quinientas páginas. (preterit)
This weekend, I read a five hundred-page novel.
De adolescente, siempre empezaba muchas novelas, pero nunca terminaba ninguna. (imperfect)
As a teenager, I began a lot of novels, but I never finished any.
El domingo, jugué al fútbol con mis amigos. (preterit)
On Sunday, I played soccer with my friends.
De niño, siempre jugaba al fútbol con mis amigos. (imperfect)
As a kid, I would always play soccer with my friends.
Ayer por la noche vi una película. (preterit)
Yesterday evening I watched a movie.
Todos los domingos veía una película con mis padres. (imperfect)
On Sundays I used to watch a movie with my parents.